Mid-wheel drive power wheelchair

ABSTRACT

A mid-wheel drive power wheelchair is provide having a pair of drive wheels on the support frame below the seat. Castor wheels are positioned behind the drive wheels and rearward of the seat. A pair of anti-tip idler wheels are positioned forward of the drive wheels on opposite sides of the frame. The anti-tip wheels are normally positioned off the ground when the drive wheels and the castor wheels are in their normal ground-engaging position. The anti-tip wheels have a resilient mounting comprising a support arm attached at a rear end to the frame. The forward positioned anti-tip wheels are movable upwardly about the attachment of the support arm, with the suspension resiliently resisting such movement, in response to the engagement with a curb or obstruction or in response to a forward pitching of the chair about the drive wheels.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application is a continuation of application Ser. No.09/370,132, filed Aug. 6, 1999, now U.S. Pat. No. 6,199,647, which is acontinuation of application No. 08/748,214, filed Nov. 12, 1996, nowU.S. Pat. No. 5,944,131, which is a continuation-in-part of applicationSer. No. 08/742,972, filed Nov. 1, 1996, now U.S. Pat. No. 6,176,335,which is a continuation-in-part of application Ser. No. 29/056,607,filed Jul. 3, 1996, now U.S. Design Pat. No. 397,645. All of these priorapplications are incorporated herein in their entirety.

FIELD OF THE INVENTION

This invention relates to powered wheelchairs for use by handicapped anddisabled persons.

BACKGROUND OF THE INVENTION

Power wheelchairs are known and have been the subject of increasingdevelopment efforts to provide handicapped and disabled persons withindependent mobility to assist handicapped and disabled persons inleading more normal lives.

Power wheelchairs known heretofore have, for the most part, resembledconventional, manual wheelchairs; indeed, many such power wheelchairshave merely been conventional wheelchairs equipped with motors. Use ofsuch power wheelchairs sometimes results in the user feeling a stigmaassociated therewith in that unthoughtful persons may view the powerwheelchair user in a quizzical or even offensive manner.

Additionally, known power wheelchairs suffer in that they tend to belarge and are not particularly maneuverable. These large, difficult tomaneuver power wheelchairs present difficulties for the power wheelchairuser in navigating within conventional dwellings which have not beenmodified to accommodate such conventional power wheelchairs.

Typical conventional rear wheel drive power wheelchairs which are littlemore than manual wheelchairs equipped with motors, have turning circlesof about 72 inches in diameter, whereas typical front wheel drive powerwheelchairs known heretofore have, for the most part, turning circles inthe neighborhood of 52 inches in diameter. These turning circles are toobig for the user of a conventional power wheelchair to reverse thewheelchair direction by turning the wheelchair around within thecorridor or hallway of a conventional office building or most homes.

Dual drive motor power wheelchairs are known; one is illustrated in U.S.Pat. No. 5,540,297. Other power wheelchairs are disclosed in U.S. Pat.Nos. 4,513,832; 4,538,857; 5,094,310; 5,145,020 and 5,366,037.

Front wheel drive power wheelchairs are sold by Permobile, Inc. inWoburn, Mass. and typically have the driving front wheels at the extremeforward end of the vehicle chassis thereby requiring substantial spacein order to turn the front wheel drive power wheelchair because the axisof rotation of the chair, upon turning, is at the mid-point of the drivewheel axes, which is at the extreme forward end of the chair.

SUMMARY OF THE INVENTION

In one of its aspects this invention provides a power wheelchair havinga frame, a seat supported by the frame, a pair of drive wheels connectedto the frame and rotatable about a transverse axis below a centralportion of the seat with the drive wheel axis and seat positioned sothat the drive wheel axis is forward of the cranial center of perceptionof a user of the power wheelchair.

In another of its aspects the invention provides a power wheelchair withthe seat positioned so that the drive wheel axis is under and perhapsonly slightly longitudinally ahead of the cranial center of perceptionof user of the power wheelchair.

In yet another of its aspects this invention provides a power wheelchairin which the drive wheel axis is at a substantially common longitudinallocation with the center of cranial perception when the wheelchair useroccupies the seat.

In yet another of its aspects this invention provides a power wheelchairin which the drive wheel axis is forward of the eyes of the wheelchairuser when the wheelchair user is seated upright in the seat.

In yet another of its aspects this invention provides a power wheelchairin which the drive wheel axis is longitudinally intermediate of thepower wheelchair user's center of cranial perception and a manualcontrol by which the power wheelchair user controls the powerwheelchair.

In yet another of its aspects this invention provides a power wheelchairin which the wheelchair user's cranial center of perception is atsubstantially a common longitudinal location with the center of gravityof the power wheelchair when the chair is unoccupied.

In yet another of its aspects this invention provides a power wheelchairin which a control joystick is used for single manual control of thewheelchair by the power wheelchair user and the drive wheel axis islongitudinally intermediate of the power wheelchair user's center ofcranial perception and the control joystick by which the powerwheelchair user controls the power wheelchair when the wheelchair useris seated upright in the power wheelchair.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a right side elevation of a front wheel drive power wheelchair in the preferred embodiment of the invention with the wheelchairuser depicted in dotted lines in an erect seated position using.

FIG. 2 is a right side view of a prototype power wheelchair manifestingaspects of the invention.

FIG. 3 is a front elevation of the power wheelchair shown in FIG. 1,manifesting aspects of the invention.

FIG. 4 is a rear elevation of the power wheelchair shown in FIGS. 2 and3, manifesting aspects of the invention.

FIG. 5 is a left side elevation of the power wheelchair shown in FIGS. 2through 4, manifesting aspects of the invention.

FIG. 6 is an top view of the power wheelchair shown in FIGS. 2 through5, manifesting aspects of the invention.

FIG. 7 is a bottom view of the power wheelchair shown in FIGS. 2 through6, manifesting aspects of the invention.

FIG. 8 is a perspective view looking at the right front of the powerwheelchair shown in FIGS. 2 through 7, manifesting aspects of theinvention.

FIG. 9 is a right side elevation of a prototype power wheelchair asshown in FIGS. 2 through 8 manifesting aspects of the invention, but notequipped with optional lights.

FIG. 10 is a front elevation of an alternate embodiment of the powerwheel chair, manifesting aspects of the invention.

FIG. 11 is a rear elevation of the alternate embodiment of the powerwheel chair shown in FIG. 10, manifesting aspects of the invention.

FIG. 12 is a left side elevation of the alternate embodiment of thepower wheel chair shown in FIGS. 10 and 11, manifesting aspects of theinvention.

FIG. 13 is a top view of a power wheelchair shown in FIGS. 10 through12, FIGS. 10 through 12 and manifesting aspects of the invention.

FIG. 14 is a perspective view looking at the right front of the powerwheelchair shown in FIGS. 10 through 13, manifesting aspects of theinvention.

FIG. 15 is a side view of the preferred embodiment of a power wheelchairillustrated in FIG. 1 and manifesting many aspects of the invention incommon with the power wheelchair embodiment as generally illustrated inFIGS. 2 through 14 as described above, but with the power wheelchairbody removed to reveal internal structure and mechanical details.

FIG. 16 is a rear view of the power wheelchair illustrated in FIGS. 1and 15 with the power wheelchair body similarly removed to revealinternal structure and mechanical details.

FIG. 17 is a top view of the power wheelchair illustrated in FIGS. 1, 15and 16 but with the power wheelchair seat and body along with a portionof the frame to reveal internal structure and mechanical details.

FIG. 18 is an exploded side view of the power wheelchair illustrated inFIGS. 1 and 15 through 17 taken in the same direction as FIGS. 1 and 15and illustrating the manner in which the power wheelchair batteries,body and seat are assembled with the power wheelchair frame and runninggear.

FIG. 19 is a partially exploded side view of the power wheelchairillustrated in FIGS. 1 and 15 through 18 taken looking in the samedirection as FIG. 17 with the batteries and wheelchair body in positionand supported by the wheelchair frame and with the wheelchair seatillustrated removed from the frame and above the frame/body assembly.

FIG. 20 is an isometric view of the frame of the power wheelchairembodiments illustrated in the figures.

FIG. 21 is a view of the wheelchair seat back looking in the samedirection as FIG. 16, illustrating wheelchair seat arm width adjustmentaspects of the invention.

FIG. 22 is a broken side view of the power wheelchair independent drivewheel suspension taken at arrows 22—22 in FIG. 17 with the drive wheelillustrated in phantom.

FIG. 23 is a broken side view of the power wheelchair independent drivewheel suspension taken at arrows 22-22 in FIG. 17 with the drive wheelillustrated in phantom, illustrating operation of the drive wheelindependent suspension upon the drive wheel encountering an obstacle.

FIG. 24 is identical to FIG. 21 and is presented for purposes of readyreference when considering FIG. 25.

FIG. 25 is a side view of the drive wheel and forward idler anti-tipwheel independent suspension apparatus as shown in FIGS. 22 through 24and illustrating operation of the anti-tip wheel suspension apparatusaspects of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to the drawings in general and to FIGS. 1 and 15 through 21 inparticular, where like numerals identify like elements, a powerwheelchair manifesting aspects of the invention is illustrated thereinand is designated generally by the number 10. Power wheelchair 10includes a frame, illustrated in FIG. 20, which is designated generallyas 12 and a seat designated generally as 14 supported by frame 12. Powerwheelchair 10 further includes a pair of drive wheels each of which hasbeen designated generally as 16, which are rotatably connected to frame12 and are rotatable about transverse axes under a central portion ofseat 14.

The inventor in developing the configuration and geometry of the powerwheelchair recognized that human beings have a center of perceptionlocated within the skull, referred to herein as the “cranial center ofperception”.

The cranial center of perception is generally perceived by a person tobe located behind one's eyes, centrally located from left to rightwithin the head and at a front to back location approximately even withthe ear opening.

The cranial center of perception provides a point of reference for allhuman beings respecting body movement. All human beings find bodymovements easier where such movements are with in the cranial center ofperception.

The inventor, recognizing the desirability of all human beings to have arelatively stationary cranial center of perception as a reference pointfor body movement, further recognized that a power wheelchair with thedriving axis of the drive wheels substantially under and supporting theweight of the wheelchair user and with the drive wheel axis (whichdefines the center of rotation for the power wheelchair when the powerwheelchair is making a u-turn, due to the action of the drive wheelswith one wheel turning forwardly and the other wheel turning rearwardly)ahead of the wheelchair user's cranial center of perception andpreferably ahead of the wheelchair user's eyes, makes the powerwheelchair easier to maneuver for a wheelchair user, especially adisabled or handicapped wheelchair user.

This has been accomplished by careful positioning of the powerwheelchair components, specifically the power wheelchair drive wheelsand the power wheelchair seat. As illustrated in FIG. 15, in the powerwheelchair the seat 14 has a cushion portion 20 for supporting awheelchair user's thighs and the lower portion of the wheelchair user'sbuttocks. The seat 14 further includes a back portion 22 for supportingthe wheelchair user's back, With the seat having cushion and backportions, the wheelchair user is positioned in a seated upright positionwhen using the wheelchair as illustrated in FIG. 1.

In this position the wheelchair user's cranial center of perception islocated above and preferably longitudinally slightly behind the axis ofrotation of the drive wheels of the power wheelchair. The drive wheels16 in general and specifically the axis 24 about which the drive wheelsare rotated is below a central portion of the wheelchair seat 14,specifically below a central portion of the cushion. This arrangementresults in the drive wheel axis being longitudinally just slightly aheadof the center of cranial perception when the wheelchair user occupiesthe wheelchair seat.

It is also desirable to have the axis of rotation of the powerwheelchair embodying the invention located longitudinally just slightlyahead of the cranial center of perception and just slightly ahead of theeye position of the power wheelchair user. In a power wheelchairembodying the invention as illustrated in the drawings, the verticalaxis or turning axis of rotation of the power wheelchair is coincidentwith the axis of rotation of the drive wheels; the drive wheels can turnthe chair about a stationary vertical axis, which intersects thetransverse or horizontal axis of rotation of the drive wheels, by havingone wheel rotate backwardly while the other wheel rotates forwardly.This causes the chair to turn about a vertical axis running through thedrive wheel axis. With this turning capability in the power wheelchairembodying the invention it is desirable for the user of the powerwheelchair to be positioned with the user's eyes slightly behind thelongitudinal location of the drive wheel axis or, less preferably, withthe user's eyes essentially at the same longitudinal position as thedrive wheel axis.

In FIG. 1 of the drawings a wheelchair user is depicted in thewheelchair embodying aspects of the invention where the wheelchair useris depicted in dotted line and is designated generally as 300. Thewheelchair user 300 is depicted seated in a wheelchair designatedgenerally as 10 which embodies the invention. In FIG. 1 the center ofgravity of wheelchair 10, which has been computed with the wheelchair inthe unoccupied condition, is designated generally 302.

In FIG. 1 a series of vertical lines have been provided to indicated therelative longitudinal position of parts of wheelchair 10 and wheelchairuser 300. These vertical lines are identified with Roman Numerals Ithrough V in FIG. 1.

Specifically in FIG. 1 the longitudinal location of the cranial centerof perception of the wheelchair user 300 is identified with verticalposition line I. The longitudinal location of the wheelchair user's eyesis denoted by longitudinal position line designated generally II in FIG.1. The longitudinal position line designated generally III in FIG. 1identifies the longitudinal location of the center of gravity 302 ofpower wheelchair 10, when power wheelchair 10 is unoccupied.

Longitudinal position line IV identifies the longitudinal location ofthe axis of rotation of the drive wheels 16 of power wheelchair 10.Longitudinal position line V in FIG. 1 identifies the longitudinallocation of the joystick controller operated by single hand operation bywheelchair user 300.

Of course, the distance between the wheelchair user's eyes and the axisof rotation of wheelchair drive wheels 16, as measured by thelongitudinal separation of position lines II and IV in FIG. 1 varieswith the position and posture and physical size of the wheelchairoccupant. When the wheelchair occupant is a male having a height ofabout 6 feet 3 inches and an average to slender torso, the longitudinaldistance between the wheelchair user's eyes and the axis of rotation ofwheelchair drive wheels 16 ranges from about (8 ½″) eight and one-halfinches down to about (1″) one inch, depending on how erect is theposture of the wheelchair user. The more erect the wheelchair user'sposture, the further the wheelchair user's eyes are longitudinallybehind the longitudinal location of the axis of drive wheels 16 ofwheelchair 10.

For a woman standing approximately 5 feet 3 inches in height, thelongitudinal separation between a female wheelchair user's eyes and theaxis of rotation of wheelchair drive wheels 16 ranges from about (6 ½″)six and one-half inches down to about (2″) two inches, depending uponthe posture and degree of forward spending of the wheelchair user. Ofcourse, the more erect the female wheelchair user sits in the wheelchairseat, the further behind, in the longitudinal direction, are thewheelchair user's eyes from the wheelchair drive wheel axis of rotation.

In most persons the distance from longitudinal separation between thewheelchair user's eyes and the wheelchair user's ear opening, which isbelieved to be substantially coincidental in longitudinal direction withthe wheelchair user's cranial center of perception, ranges from about 2inches to about 3 inches, depending upon whether the wheelchair user ismale or female and the size of the wheelchair users head. Hence for themale and female wheelchair user's noted above, the wheelchair user'scenter of cranial perception is always at least about 3 inches or morebehind the longitudinal location of the wheelchair drive wheel axis ofrotation 24.

In the preferred embodiment of the wheelchair, the joystick ispositioned longitudinally, as indicated by longitudinal position line V,forward of the wheelchair axis of longitudinal axis of rotation by adistance of about 9 inches.

The center of gravity 302 of wheelchair 10 when wheelchair 10 is emptyin the preferred embodiment illustrated in FIG. 1 is about 3.1 incheslongitudinally behind the axis of rotation 24 of drive wheel 16. Thislongitudinal location of the wheelchair center of gravity when empty isindicated by longitudinal position line III in FIG. 1.

This arrangement and geometry provides the user of power wheelchair 10with a vehicle which reacts very much like the wheelchair user's ownbody. The pivot axis of the power wheelchair is much the same asexperienced by a power wheelchair user in navigating normal hallwaysturns and the like. Specifically, the pivot point for such turning isnot behind the user of the power wheelchair when seated in the powerwheelchair. The pivot point being coincident with the axis of rotationof the drive wheels of power wheelchair 10 allows the power wheelchairuser to drive the power wheelchair and essentially to react to what theuser's eyes see. There is very little necessity to align the powerwheelchair with narrow openings in advance; the power wheelchair can bedriven much like a conventional automobile or like a runner runs withthe joystick responding to the power wheelchairs user's eyes and whatthe power wheelchair user sees. The response in that regard is ergonomicin that the power wheelchair user may rely on what he or she sees aheadand can guide the power wheelchair accordingly with the power wheelchairuser pivoting about an axis very nearly coincident with the axis aboutwhich the power wheelchair user would pivot the user's torso if thepower wheelchair user could walk. This gives a very lifelike perceptionto the power wheelchair user when guiding the power wheelchair, unlikepower wheelchairs known heretofore.

All of this is accomplished by the geometry of the power wheelchair, theseat, the arrangement of the wheels relative to the seat and thepositioning of the power wheelchair user on the seat relative to thecenter of gravity of the power wheelchair, the drive wheels and the axisof rotation thereof.

As is apparent from the drawings, drive wheels 16 of power wheelchair 10are connected to frame 12 so that each drive wheel 16 rotates about atransverse axis which is below a portion of a cushion portion 20 of thepower wheelchair seat designated generally 14, and specifically belowthe portion of cushion 20 which supports the power wheelchair occupant'sthighs. As is apparent from the drawings, drive wheels 16 are rotatableabout transverse axes which are slightly forward of the longitudinalmid-point of the wheelchair seat 14 and specifically are rotatable abouttransverse axes which are slightly forward of the longitudinal mid-pointof the seat cushion 20.

Seat 14 is preferably mounted on frame 12 proximate the longitudinalmid-point of frame 12. As is further apparent from the drawings, drivewheels 16 are connected to frame 12 and rotatable with respect theretoabout a transverse axis which is under a central portion of frame 12which is adapted to support seat 14. As is further apparent from thedrawings, drive wheels 16 which are rotatably connected to frame 12 arerotatable about a transverse axis which is preferably under a centralportion of seat 14. The axes 24 of drive wheels 16 is preferablylongitudinally between the mid-point of seat 12 and the seat forwardextremity and is preferably closer to the mid-point of seat 14 than tothe longitudinal extremity thereof.

Cushion portion 20 of seat 14, is the portion of cushion 20 whichsupports a seat occupant's buttocks and thighs. The forward portion ofthe seat cushion supports the occupant's thighs.

Frame 12 is illustrated isometrically in FIG. 20 and has an upperrectangular frame portion designated generally 60 which includes leftand right longitudinally extending tubular upper members 62 and forwardand rear transversely extending tubular upper members 64. Members 62 and64 are preferably welded together. As illustrated in FIG. 20, frame 12is preferably fabricated of hollow tubular rectangular cross-sectionsteel members which are welded together.

Frame 12 further includes a pair of forward vertically downwardlyextending members 66 which are preferably welded to and extenddownwardly from forward extremities of longitudinally extending tubularupper members 62 as illustrated in FIG. 20. A transversely extendingforward lower cross-member 68 is preferably welded to lower extremitiesof downwardly extending members 66 and extends therebetween.

Preferably welded to and extending vertically downwardly from the centerof a rear transversely extending tubular upper member 64 is a rearvertically downwardly extending member 74 forming a portion of frame 12.

A pan 70 for carrying electrochemical means, for powering the motorsrotating drive wheels 16, in the form of one or more batteries 72, ispreferably welded along its forward edge to forward transverselyextending lower cross-member 68. A rear transversely extending lowercross-member 69 is welded to the lower extremity of rear verticallydownwardly extending member 74. The rear edge of pan 70 is welded to theforward facing surface of rear transversely extending lower cross-member69, which is of length equal to the width of pan 70; rear transverselyextending lower cross-member 69 is not as long in the transversedirection as transversely extending cross-members 64, 68.

Longitudinally extending tubular upper members 62 preferably haveapertures 174 formed therein, preferably by drilling or stamping.Apertures 174 receive front and rear upwardly extending seat supportmembers which provide for manual height and tilt adjustment of the powerwheelchair seat without use of tools. Between apertures 174 inlongitudinally extending tubular upper members 62 are apertures 176which are provided for mounting a spring-strut portion of theindependent drive wheel suspension means of wheelchair 10.

Frame 12 further includes upper and lower longitudinally extendingcentral tubular members 178, 179 respectively. Upper member 178 ispreferably welded to and extends rearwardly from the center of rearupper transversely extending tubular member 64. Lower longitudinallyextending central tubular member 179 is preferably welded to and extendsrearwardly from the center of the rearwardly facing surface of reartransversely extending cross-member 69, immediately below the point ofwelding connection between rear vertically downwardly extending member74 and rear transversely extending lower cross-member 69.

Extending vertically between longitudinally extending central upper andlower tubular members 178, 179 is outboard vertically extending tubularmember 180, which is preferably secured to the rear extremities oflongitudinally extending members 178, 179 by welding.

Frame 12 further includes a longitudinally extending forward tubularextension member 198 which is secured to transversely extending lowerforward cross-member 68 preferably at the center thereof, preferably bywelding.

Frame 12 yet further preferably includes a forward upstanding extensionmember 200 secured by welding to the forward extremity of forwardtubular extension member 198 and extending transversely upwardlytherefrom as illustrated in FIG. 20.

Frame 12 still yet further preferably includes a pair of eye portionsdesignated generally 82 in FIG. 20 which preferably consist of a pair ofparallel plates affixed to rearwardly facing surfaces of respectivevertically downwardly extending forward members 66. The platesconstituting eye portions 82 include apertures 84 formed therein whereapertures 84 are aligned in respective ones of plates forming eyeportion 82 so that each pair of plates may receive shafts forindependent suspension of drive wheels from eye portions 82 as describedbelow. Eye portions 82, specifically the parallel plate portionsthereof, are preferably formed from single pieces of metal by bendingthe pieces of metal into a U-shaped bracket where the upstandingportions of the “U” define the plates of eye portion 82. The preferredone piece, U-shaped bracket construction of eye portion 82 is visible inFIG. 17.

Frame 12 preferably yet further includes a pair of forwardly extendingpedestals 202 which are secured to forwardly facing surfaces of forwardvertically downwardly extending members 66, preferably by welding.Pedestals 202 are preferably secured at positions on members 66 somewhatbelow the upper extremities thereof, as illustrated in FIG. 20.Forwardly extending pedestals 202 provide points of attachment forspring-strut-beam combinations 44 which govern arcuate upward movementof anti-tip wheels 42 relative to frame 12. The spring-strut-beamcombinations 44 and anti-tip wheels 42 are not illustrated in FIG. 20but are shown in FIGS. 15, 17 through 19 and FIGS. 22 through 25.

Pan 70 is preferably welded to a forward portion of rear verticallydownwardly extending member 74 as well as to a rear portion oftransversely extending lower tubular cross-member 68. The preferablywelded, box-like construction of frame 12 as illustrated in FIG. 20provides a rigid, high strength frame for power wheelchair 10.

As illustrated in FIGS. 1 and 15, drive wheels 16 are preferablyoriented and positioned respecting frame 12 so that the axes of rotationof drive wheels 16 are between the longitudinal mid-point and forwardextremity of seat 14. Most desirably the axis of rotation of drivewheels 16 is closer to the mid-point of seat 16 than to the forwardlongitudinal extremity of seat 16.

Seat 14 preferably includes a cushion portion 20 and a back portion 22.In FIG. 15 the longitudinal mid-point of seat 14, specifically thelongitudinal mid-point of seat cushion 20, is identified by the numeral26. The forward longitudinal extremity of seat 14 is identified by thenumeral 28 while the rear longitudinal extremity of seat 14 andspecifically seat cushion 20 is identified by the numeral 30. The axisof rotation of drive wheel 16 is identified by the numeral 24. In FIG.15 the longitudinal mid-point 26 of seat 14, specifically of cushion 20of seat 14, is indicated by both a lead line from an indicator numeraland by a dotted vertical line. The longitudinal location of the axes ofrotation of drive wheels 16 is indicated by dotted vertical line 27 inFIG. 15.

Power wheelchair 10 further includes at least one battery, which hasbeen designated generally 32, carried by frame 12, specifically by pan70 as illustrated in FIG. 16. Power wheelchair 10 further includesmotors 76 powered by one or more batteries 32 for driving drive wheels16. Each drive wheel 16 preferably has an associated drive motor 76associated therewith as illustrated in FIG. 17.

As best illustrated in FIGS. 16 and 17, power wheelchair 10 furtherincludes a pair of idler wheels 18 which are the rear ground engagingwheels of power wheelchair 10; drive wheels 16 are the forward groundengaging wheels of power wheelchair 10. In the preferred embodiment ofthe invention rear ground engaging wheels 18 are caster-type wheels.

The mid wheel drive power wheelchair 10 in the preferred embodiment ofthe invention as illustrated in the drawings, particularly in themechanical configuration illustrated in FIGS. 1 and 15 through 20,provides a mid wheel drive power wheelchair with the drive wheelspositioned towards the longitudinal center of the power wheelchairvehicle. This configuration concentrates, and is believed to effectivelymaximize, weight placed on drive wheels 16. A power wheelchairconstructed in accordance with FIGS. 15 through 21 and having dimensionsas set forth herein as representing the preferred embodiment of theinvention, when carrying a 160 pound passenger has about eighty-twopercent (82%) of the total weight of the power wheelchair and passengercarried by drive wheels 16.

The configuration illustrated in the drawings, particularly in FIGS. 1and 15 through 20, positions a large amount of total chair and passengerweight over drive wheels 16, which provides several benefits: Overalltraction is increased. With increased traction, better obstacle climbingability results, increasing overall capability and usability of thepower wheelchair. Additionally, with increased traction, deceleration ismore positive and more predictable. This is an important safety factorin light of the fact that most, if not all of the users of powerwheelchairs manifesting invention will be handicapped and disabledpersons.

With increased traction also comes superior straight line stability ofthe power wheelchair. The mass centralization created by the batteryplacement and the user position in the power wheelchair contributes tostraight line stability by reducing the “pendulum effect” present inmany prior art front wheel drive power wheelchairs, where the user andbattery weight are both behind the pivot axis of the drive wheels. The“pivot axis” of the drive wheels means the vertical axis about which thepower wheelchair turns when the power wheelchair is turned in as tight aturning radius as possible.

Increased traction further results in extremely accurate response by thepower wheelchair to the control joystick inputs provided by thewheelchair user. This translates into more predictable and positivehandling and a much easier learning curve for the power wheelchair userwhen the user is first becoming accustomed to the power wheelchair.

Yet another benefit of the geometry and configuration of the componentsin the preferred embodiment of the invention, as illustrated in FIGS. 1and 15 through 20, is an extremely tight turning radius. This allows theuser of the power wheelchair to gain access to and to turn around inconfined areas such as those encountered in hallways, bathrooms, smallkitchens and office areas. In the preferred embodiment of the inventionas illustrated in FIGS. 1 and 15 through 19 the power wheelchair has anextremely small footprint. Placement of the drive wheels near the centerof the power wheelchair results in the power wheelchair having a turningradius of only nineteen and one-half inches (19 ½″) in the preferredembodiment of the invention.

As illustrated in FIGS. 17 and 18, idler wheels 18 located at the rearof power wheelchair 10 are mounted to frame 12 and are pivotable aboutvertical axes identified by the numeral 36 in FIG. 15. Idler wheels 18are not powered and are desirably caster-type wheels. Idler wheels 18are connected to frame 12 behind drive wheels 16 and, preferably, behindseat 14.

As illustrated in FIG. 16 rear idler wheels 18 are connected to atransverse beam 38 via U-shaped spindles 142 which connect to transversebeam 38 via conventional bearing assemblies 144. With this arrangementU-shaped spindles and hence rear idler wheels 18 are pivotable aboutvertical axes 36 upon wheelchair 10 turning and/or one of rear idlerwheels 18 encountering an obstacle such as illustrated in FIG. 16.

Rear idler wheels 18 are rotatably mounted within U-shaped spindles 142for rotation within the spindles about horizonal axes 50 as illustratedin FIG. 17.

As illustrated in FIG. 16, transverse beam 38 is pivotally connected toframe 12, specifically to the upper portion of outboard verticallyextending tubular rear frame member 180. The point of pivotal connectionof transverse beam 38 to vertically extending outboard rear frame member180 is designated 204. Pivotal connection provided at 204 is effectuatedusing conventional bearings together with a pin journeled in the bearingfor pivotally connecting beam 38 to vertical extending frame member 180.Member 180 extends upwardly from a lower longitudinally extending framerear extension member 179 which in turn extends rearwardly from a reartransversely extending lower cross-member 69 as illustrated in FIG. 20.

Pivotal mounting of transverse beam 38 to vertically extending rearframe member 180 provides a smoother ride in the event wheelchair 10encounters a bump. As illustrated in FIG. 16, where the right hand oneof rear idler wheels 18 has been illustrated encountering an obstacle,as right hand idler wheel 18 rides over the obstacle, beam 38 rotatesabout pivotal connection 204 as indicated by double ended arrows G inFIG. 16. Vertical displacement of right hand idler wheel 18 is depictedby double ended arrow H in FIG. 15 where this displacement iseffectuated by idler wheel 18 encountering the obstacle.

As illustrated in FIG. 17 power wheelchair 10 includes two motors fordriving respective drive wheels 16. These motors are designatedgenerally 76 and are each within a rigid housing which houses, inaddition to a motor 76, a transmission 78 for transferring drivingrotation from an output shaft of motor 76 to an associated drive wheel16. The drive wheel/motor transmission combination housing isindependently suspended from frame 12. Hence each drive wheel 16 is freeto move with respect to frame 12 upon encountering an obstacle withoutthe remaining drive wheel 16 moving relative to frame 12. There is nocommon single axle for drive wheels 16 in the preferred embodiment ofthe invention.

To facilitate independent suspension of each drivewheel/motor/transmission combination respecting frame 12, frame 12further includes eye portions 82 which are preferably fixedly connected,preferably by welding, to forward vertical members 66 of frame 12 asillustrated in FIG. 20. Eye portions 82 have apertures 84 formedtherein.

Motor 76 and transmission 78 are available from Rockwell Automation inEden Prairie, Minn. Transmission 78 is a right angle worm drive servingto change the axis about which the driving rotation is provided by motor76. Specifically, motor 76 is positioned so that the motor output shaftextends longitudinally. Associated transmission 78 through the rightangle worm drive provides the driving rotation output via axles, notnumbered in the drawings, on which connecting to drive wheel 16.

Each motor 76 drives an associated drive wheel 16 via an associatedtransmission 78. A shift lever 79 extending out of transmission 78 maybe rotated to disengage transmission 78 thereby providing free wheeloperation of drive wheels 16. When body 34 is in place on frame 12,shift levers 79 protrude through apertures in body 34 thereby permittingfacile switchover from driven to freewheeling operation of drive wheels16 by the wheelchair operator merely twisting shift lever 79.

Motor 76 and transmission 78 are rigidly connected by motor/transmissionhousing 80. Motor and transmission housing 80 is preferably glass fillednylon, which is extremely strong, provides extremely quiet operation andincludes an ear portion 86 extending forwardly therefrom as illustratedin FIG. 22. Ear portions 86 include apertures 88 sized and positioned tobe congruent with apertures 84, as illustrated in FIG. 20, so thatapertures may be aligned. A pivot pin 90 fits within aligned aperturesand permits rotation of housing 80, and hence motor 76 and transmission78 housed therewithin, relative to frame 12 upon an associated drivewheel 16 encountering an obstacle. Once a drive wheel 16 encounters anobstacle and moves upwardly as indicated by arrow C in FIG. 23, housing80, having motor 76 and transmission 78 therewithin, rotates upwardly,about a pivot defined by pin 90, relative to frame 12. Bearings areprovided at pin 90 to provide for free rotation of motor/transmissionhousing 80 relative to frame 12 about pin 90.

Further forming a part of the drive wheel independent suspensionapparatus is a shaft-spring combination, which combination has beendesignated 95 in the drawings and is best illustrated in FIGS. 22through 25. A shaft 92 is pivotally affixed to a web 170 formedintegrally as a part of housing 80. Most preferably two parallel webs170 are provided which are longitudinally commonly position andtransversely aligned such that only a single web 170 is illustrated inFIGS. 22 through 25; the second, remaining web is immediately behind thevisible web 170 and is spaced therefrom.

The pivotal connection of the shaft to web 170 is provided by means of ashaft which extends between web 170 and the web which is hidden fromview. The shaft is preferably secured to a fitting which fits rotatablyon the shaft extending between web 170 and the unnumbered web hiddenfrom view. The shaft extends upwardly from connection with web 170through an aperture, not shown in FIG. 22, formed in the lower surfaceof upper longitudinally extending member 62 of frame 14. A coil spring94 is wrapped around the shaft and at one end preferably abuts web 170or is fixedly connected to the shaft or a sleeve about the shaft whichextends between web 170 and the web which is hidden from view in FIG.22. Spring 94 at its remaining end abuts, but is not fixed to, the lowersurface of longitudinally extending member 62.

Hence, when an associated drive wheel 16 encounters an obstacle asillustrated in FIG. 23 and moves upwardly as indicated by arrow C inFIG. 23, housing 80 pivots about pin 90, moving in a clockwise directionas indicated by arrow D in FIG. 23 as described above. This upwardpivotal movement of housing 80 compresses spring 94 as web 170 movesupwardly thereby reducing distance from web 170 to the lower surface oflongitudinally extending member 62. Spring 94, being compressed, exertsdownward force on web 170 and hence on housing 80. This force urgeshousing 80 to rotate counter-clockwise respecting pivot pin 90 asindicated by arrow E in FIG. 23 when drive wheel 16 has passed over theobstacle and moves downwardly as indicated by arrow F in FIG. 23.

The shaft is slidably retained within fitting in the bottom wall oflongitudinally extending tubular member 62. A nut threadedly engages theshaft on the upper side of the bottom wall of longitudinally extendingtubular member 62. Sliding passage of the shaft through the bottom wallof longitudinally extending tubular member 62 permits the shaft to risevertically in response to an associated drive wheel encountering anobstacle such as illustrated in FIG. 22. A clearance aperture cut in theupper wall of longitudinally extending member 62, immediately above theposition of residence of the nut retaining the shaft, permits upwardmovement of the shaft upon the associated drive wheel encountering anobstacle without the shaft interfering with frame 12 and particularlywith longitudinally extending member 62. The downwardly facing surfaceof the lower wall of longitudinally extending member 62 contacts theupper end of spring 94, thereby precluding upward movement of spring 94and causing spring 94 to compress upon upward movement of web 170.

Power wheelchair 10 further preferably includes a body 34 which not onlyprovides a decorative, aesthetically pleasing appearance for powerwheelchair 10 but also protects the wheelchair user from batteries 32and the electrical connections between batteries 32 and motor 76. Body34 further provides protection for batteries 32 and, to some extentmotors 76, from spills of liquids, bodily fluids and the like.

Body 34 preferably includes a central generally planar portion which,when body 34 is in place on frame 12, overlies batteries 32. Thiscentral planar portion is designated 172 and is visible in FIGS. 18 and19 and, to a lesser extent, in FIGS. 2, 5, 8, 9, 10, 12 and 14.

As illustrated in FIGS. 2 through 5, 8 through 12, 14, 18 and 19, body34 preferably further includes fender portions 126 which preferablysubstantially wrap around upper semi-circular portions of respectivedrive wheels 16. Fenders 126 preferably extend outwardly preferably overat least major portions of the width of associated drive wheels 16, tofit closely about the drive wheel portions which are enveloped byrespective fenders 126.

Body 34 may further include a rearwardly facing openable bonnet portiondesignated generally 128 in the drawings and best seen in FIGS. 4 and11. There may be further provided as a portion of body 34 a planarportion 130 extending downwardly from openable bonnet 128 for preferablyat least substantially concealing the rear suspension of wheelchair 10,especially transverse beam 38 from which rear idler wheels 18 aresuspended.

As illustrated in FIGS. 2, 4, 5, 8, 9, 11, 12, 14, 18 and 19 body 34preferably further has rear corner portions 132 which are preferablyrounded and also extend over the suspension gear for rear idler wheels18, specifically transverse beam 38 and U-shaped spindles 142. Roundedrear corners 132 and fender portions 126 preferably transition tocentral planar portion 124 via at least partially concave transitionportions 134 which have apertures formed therein for passagetherethrough of upstanding front and rear seat support members 96, 98and additional apertures therein for passage therethrough of shiftlevers 79.

As illustrated in FIGS. 2, 3, 5, 6, 8 through 10 and FIGS. 12 through14, body 34 preferably yet further includes and has formed therein apair of forwardly facing integral bumper members 138 lying over andprotecting the spring-strut-beam combinations designated generally 44which govern arcuate upward movement of forward anti-tip wheels 42relative to frame 12. Body 34 preferably further includes a planarportion extending between forward bumper members 138 where such planarportion is designated 136 and provides a kick panel for the powerwheelchair occupant. The kick panel is designated 136 and is illustratedin FIGS. 3, 6, 7 and 10 of the drawings.

Body 34 preferably rests directly on frame 12. Body 34 is preferably asingle molded piece of high impact plastic and is exceedingly light.Hence, body 34 may be manually directly lifted off of frame 12 once seat14 has been removed, without use of tools. Because body 34 fits closelyabout frame 12 and is effectively contoured to the shape of frame 12 andthe associated members by which the rear idler wheels, the forwardanti-tip wheels and the other structure are connected to frame 12, body34 need not be fixed in any way to frame 12. In the preferred embodimentof the invention the power wheelchair operates exceedingly well withbody 34 resting on but not secured to frame 12.

If desired body 34 may also be lightly retained in place on frame 12 bymating pads of respective hook and eye material, such as that sold underthe trademark Velcro, affixed to upwardly facing surfaces oflongitudinally extending tubular member 62, preferably at the forwardextremities thereof. Corresponding mating pieces of Velcro are affixedto the under surface of body 34 at planar portions thereof which restdirectly on the forward extremities of longitudinally extending tubularmembers 62. Use of the hook and eye material attachments reduces andindeed effectively eliminates any rattles which might otherwise occur asbody 34 which moves slightly relatively to frame 12 during operation ofpower wheelchair 10.

Forward anti-tip idler wheels 42 do not normally contact the ground orother surface on which wheelchair 10 operates. Anti-tip wheels 42 aremaintained above the ground and provide protection against tipping inthe event of forward pitching of wheelchair 10 due to encounter with anobstacle, traverse of a significant downgrade and the like. Theoff-the-ground, anti-tip positioning of idler wheels 42 is illustratedin FIGS. 1 and 15.

Anti-tip idler wheels 42 are connected to frame 12 via aspring-strut-beam combination which is designated generally by thenumeral 44 in the drawings, specifically in FIGS. 15 and 22 through 26.Each spring-strut-beam combination 44 includes a U-shaped spindle 228;one leg (of the U-shape) of one of spindles 228 is shown in side view inFIG. 15. The U-shape of spindles 228 is readily apparent from FIG. 3, 8,10 and 14.

U-shaped spindles 228 are preferably fabricated by welding rectangularcross-section tubular stock between two parallel plates with the tubularstock forming the base of the U. Spindles 228 are preferably pivotallyconnected to frame 12 preferably using screw-bolt assemblies. The sideplate portions of spindles 228 fit pivotally on either side about thelower portions of respective downwardly extending vertical tubularmembers 66, which are illustrated in FIGS. 20 and 22 through 25. The nutand bolt pivotal connections of U-shaped spindles 228 to verticallydownwardly extending members 66 are depicted schematically by indicatornumeral 230 in FIGS. 22 through 25.

Extending between the legs of U-shaped spindles 228 are shafts 234.Mounted on shafts 234, via passage of shafts 234 through bores formedtherein, are cylindrical spring support bases 236, upper mostextremities of which are visible in FIGS. 22 through 25. Secured to andextending from cylindrical spring support bases 236 are upstandingshafts 238, the upper extremities of which extend through and areslidably retained within fittings which are resident within apertures,which have not been numbered in the drawings, formed in horizontallyextending planar portions of forwardly extending pedestals 202, whichare visible in FIG. 20 as well as in FIGS. 22 through 25. One ofupstanding shafts 238 has been identified by a lead line extending tothe vertical extremity thereof in FIGS. 24 and 25.

With this arrangement, upon an anti-tip wheel 42 encountering anobstacle or upon wheelchair 10 pitching forwardly, as depictedschematically in FIG. 25, anti-tip wheels 42 move arcuately, togetherwith U-shaped spindle 228, as they pivot about pivotal connection 230relative to frame 12. This pivotal motion is denoted by double endedarrow Q in FIG. 25. As the illustrated anti-tip wheel 42 and U-shapedspindle 228 pivot about connection point 230, upward movement of spindle228 causes the distance between shaft 234 and pedestal 202 to decrease,thereby compressing spring 236 in the direction indicated by doubleended arrow R in FIG. 25.

Compression of spring 236 provides a cushioning effect when anti-tipwheels 42 contact an obstacle or contact the ground due to forwardpitching of power wheelchair 10 as illustrated schematically in FIG. 25.The suspension of anti-tip wheels 42 provided by spring-strut-beamcombination 44, where this combination is defined by spring 236 with the“strut” being provided by shaft 238 and the “beam” being provided byspindle 228, connects anti-tip idler wheels 42 to frame 12 for arcuateupward motion relative to frame 12 upon tipping of power wheelchair 10or contact of anti-tip wheels 42 with an above-grade obstacle.

A nut 240 mounted on the threaded portion of shaft 234 extending abovepedestal 202 permits selectable compression of spring 236 therebyproviding adjustment of the spring force applied to anti-tip secondidler wheels 42 to resist arcuate upward movement thereof upon forwardtipping of power wheelchair 10 or upon wheels 42 encountering anobstacle. Rotation of nut 240 also adjusts the distance at which wheels42 are from the ground.

The tight maneuverability feature of the power wheelchair achieved bylocating the drive wheels, which are front wheel drive wheels, close tothe longitudinal center of the power wheelchair, while having manyattendant advantages as described above, has a minor disadvantage inthat there is a slight tendency to tip if a significant obstacle isencountered when the chair is decelerating or traveling forwardlydownhill.

The slight tendency towards forward tipping is counteracted by thespring loaded anti-tip wheels 42 located in front of each drive wheel16. Spring loading of anti-tip wheels 42 is accomplished via springs 236forming portions of spring-strut-beam combinations 44 biasing anti-tipwheels 42 downwardly towards the ground. When choosing the rate forthese springs used in connection with anti-tip wheels 42, compromise isrequired between a spring rate stiff enough to resist forward tippingupon deceleration of the power wheelchair yet light enough to allow thepower wheelchair to overcome minor obstacles such as inclinetransitions, curves or other uneven terrain.

Longitudinal distance between the position of the pivot 90 of housing80, and the drive axle for the associated drive wheel, where the driveaxle emerges from housing 80 to drive the associated drive wheel 16, ispreferably in the neighborhood of from two and three quarters (2 and ¾)to three (3) inches, measured longitudinally. This pivotal connection ofthe drive motor/transmission housing 80 to frame 12 via the pivotalconnection of ear portion 86 with eye portion 82 provides the activeindependent suspension system for the combination of motor 76 andtransmission 78 in housing 80 and associated drive wheel 16.

Upon power wheelchair 10 accelerating forwardly, the rear of motor 76tends to drop and housing 80 tends to pivot downwardly about pivot pin90 residing in the apertures formed in the eye portion 82 of frame 12and ear portion 86 of housing 80 respectively. Conversely, when thepower wheelchair decelerates as the power wheelchair user allows thecontrol joystick to return to the center position, the rear of motor 76tends to move upwardly as housing 80 tends to rotate about the pivotpoint defined by pivot pin 90.

The independent suspension of the drive motor/transmission housing 80with an associated drive wheel 16, provides some interaction withanti-tip wheels 42 and minimizes the range of spring constants whichmust be considered in the course of the design compromise.

Preferably cushion portion 20 of seat 14 is 17 inches wide. Back portion22 of seat 14 is preferably 16 inches high in the embodiment illustratedin FIGS. 1 and 15 through 21. In the embodiment illustrated in FIGS. 2through 14 back portion 22 of seat 14 is 18 or 19 inches high.

In the embodiment illustrated in FIGS. 1 and 15 through 21 thelongitudinal length of cushion portion 20 of seat 14 is preferablybetween 16 and 18 inches, most preferably 16 inches.

Drive wheels 16 are preferably fourteen inch (14″) diameter drivewheels. Rear idler wheels 18 are preferably eight inch (8″) diameterwheels. Second idler wheels 42 which are the forward anti-tip wheels,are preferably six inch (6″) diameter. Power wheelchair 10 preferablyhas an overall length of thirty-nine and one-half inches (39½″) and awidth of twenty-five and four-tenths inch (25{fraction (4/10)}″). Thisresults in the vehicle having a turning radius of nineteen and one-halfinches (19½″) permitting power wheelchair 10 to be used easily in anindoor environment, including those that have not been modified toaccommodate handicapped personnel and conventional wheelchairs.

Power wheelchair control is effectuated utilizing a joystick controllerdesignated generally 196 in the drawings. The joystick controller issupplied by Penny & Giles in Cristchurch, England, and is customprogrammable and adjustable to provide variable sensitivity for theuser. During operation of the power wheelchair of the invention thejoystick controller is programmed so that direction or steering is thefirst correction provided in response to movement of the joystick. Amicroprocessor is provided which further controls operation of the powerwheelchair of the invention by increasing or decreasing speed accordingto a logarithmic function of the position of the joystick; this speedcorrection is provided after steering correction.

The geometry provided by the arrangement of the frame, seat, drivemotors, drive wheels, idler wheels and tilt wheels provides outstandinghandling and control as a result of weight distribution of the vehiclebeing very low.

In the preferred embodiment of the invention illustrated in FIGS. 1 and15 through 21, with body 34 in place on frame 12 the center of gravityof power wheelchair 10 when power wheelchair 10 is empty is about oneand nine-tenths inches (1 and {fraction (9/10)}″) above the axis ofrotation of drive wheels 16. Further, the center of gravity ofwheelchair 10 when wheelchair 10 is empty in the preferred embodimentillustrated in the aforementioned drawing figures is about three andone-tenth (3.1) inches behind the axis of rotation of drive wheels 16.

With the aforementioned dimensions of the preferred embodiment, whenwheelchair 10 is empty, the center of gravity of wheelchair 10 is about15.2% of the longitudinal distance the axis of drive wheels 16 and rearidler wheels 18.

The longitudinal location of the center of gravity found to be 15.62inches behind the axis of the forward anti-tip wheels 42, was locatedexperimentally by hanging a fully equipped wheelchair in the preferredembodiment of the invention as illustrated in the aforementioneddrawings, from the ceiling by its upper frame members. The connectionpoint to the upper frame members was moved forward and backwards,longitudinally, in small increments until the balance point was located.

The vertical component of the center of gravity was determined using theFederal Aviation Administration approved method which is commonly usedas a preflight determination of changes in center of gravity of aircraftdue to fuel, baggage, passenger and pilot loading and load changes. Thismethod is set forth in many references, one of which is the AnInvitation To Fly—Basics for the Private Pilot by Dennis Glaeser,Sanford Gum and Bruce Walters published by Wadsworth Publishing Companyof Belmont, Calif., Copyright 1989, the disclosure of which is herebyincorporated by reference. Table 1 provides the results of thecalculations using this method to determine the vertical location of thecenter of gravity of the power wheelchair in the preferred embodiment.

TABLE 1 Items Aft Datum Weight Moment Back Rest 28.25 4.44 125.43 ArmRest w/ 22.64 6.78 153.4992 Upright & Joystick Arm Rest w/Upright 22.644.9 110.936 Foam Base Cushion 16.82 4.25 71.485 Seat Frame w/PlasticBase 14.78 7.32 108.1856 Rear Articulation Arm 9.1 3.22 29.302 Charger8.97 3.64 32.6508 Rear Forks 8.77 1.56 13.6812 Frame Center 8.27 28.06232.0562 Batteries 7.7 96.8 745.36 Module 7.58 4.28 32.4424 Main DriveWheels 6.55 18.4 120.52 Drive Motors 5.32 30.24 160.8768 Front Forks4.37 3.28 14.3336 Footrest 3.79 3.2 12.128 Rear Wheel Casters 3.6 2.649.504 Front Wheels 3.5 1.16 4.06 Totals 188.65 224.17 1976.4548 CenterOf Gravity Aft Datum: 8.816767632

What is claimed is:
 1. A power wheelchair comprising: a. a supportframe; b. a seat mounted to the support frame, the seat having a seatportion for supporting a person's thighs and buttocks and a back portionfor supporting a person's back when seated upright in the seat; c. apair of ground engaging drive wheels on the support frame rotatableabout transverse axes, the drive wheels positioned below said seatportion and forward of the back portion; d. at least one motor forrotatably driving the drive wheels to provide powered movement of thewheelchair, the motor being supported by the frame; e. at least onebattery for supplying power to the at least one motor, the batterysupported on the frame; f. at least one normally ground-engaging castorwheel connected to the frame behind the drive wheels and rearward of theback of the seat; g. one or more anti-tip idler wheels positionedforward of the drive wheels and the seat, wherein all of the wheelspositioned forward of the drive wheels are normally positioned off theground when the drive wheels and the at least one castor wheel are intheir normal ground-engaging position on level ground; and h. aresilient mounting for said one or more anti-tip idler wheels, saidmounting comprising a support arm attached at its rear end to thesupport frame and carrying said one idler wheel at its front end; i. oneor more anti-tip idler wheels being movable upwardly on its support armagainst the resilience of its mounting forward of the attachment to thesupport frame.
 2. The wheelchair according to claim 1, wherein there aretwo anti-tip idler wheels, each anti-tip idler wheel supported by arespective resilient mounting, the support arm of each resilientmounting pivotally attached at one location to the support frame andeach resilient mounting including a spring attached to the support frameat a second location, each resilient mounting adapted to permitdeflection of the associated anti-tip idler wheel about the pivotalattachment, the spring adapted to resiliently compress in the event ofsuch deflection and to bias the anti-tip idler wheel back to itsnon-deflected position.
 3. The wheelchair according to claim 1, whereinthere are two one motor engaged with each drive wheel.
 4. The wheelchairaccording to claim 1, wherein each drive wheel is mounted to the supportframe through a resilient connection which permits the drive wheel toresiliently pivot about a fixed point on the support frame.
 5. Thewheelchair according to claim 1, wherein there are two batteries.
 6. Thewheelchair according to claim 1, wherein there are two rearground-engaging castor wheels, and wherein the rear castor wheels arepivotally connected to the frame.
 7. A power wheelchair comprising: asupport frame; a seat mounted to the support frame, the seat having aseat portion for supporting a person's thighs and buttocks and a backportion for supporting a person's back when seated upright in the seat;a pair of normally ground engaging drive wheels on the support framerotatable about transverse axes, the drive wheels positioned below theseat portion and forward of the back portion; motor means for rotatablydriving each of the drive wheels to provide powered movement of thewheelchair, the motor supported on the frame; controller means forcontrolling the driving of the drive wheels by their respective motors;at least one battery for supplying power to the at least one motor, thebattery supported on the frame; at least one normally ground-engagingcastor wheel connected to the frame behind the drive wheels and rearwardof the back of the seat; a pair of anti-tip idler wheels positionedforward of the drive wheels and the seat, the pair of anti-tip wheelscomprising all of the wheels forward of the drive wheels and therespective anti-tip wheels positioned on opposite sides of the frame andpositioned off the ground when the drive wheels and the castor wheel arein their normal ground-engaging position on level ground; and aresilient mounting for each said anti-tip idler wheels, each saidmounting comprising a support arm attached at a rear end to the frameand the anti-tip idler wheel rotatably supported at a front end of thesupport arm, the idler wheel being movable upwardly against theresilience of mounting, the upward movement being forward of theattachment of the support arm to the frame.
 8. The wheelchair accordingto claim 7, wherein each said support arm is pivotally attached at onelocation to the support frame and each said resilient mounting includesa spring attached to the support frame at a second location, thesuspension adapted to permit deflection of the anti-tip idler wheelabout the pivotal attachment, the spring adapted to resiliently compressin the event of such deflection and to bias the anti-tip idler wheelback to its non-deflected position.
 9. A power wheelchair comprising: asupport frame; a seat removably mounted to the support frame, the seathaving a seat portion for supporting a person's thighs and buttocks anda back portion for supporting a person's back when seated upright in theseat; a pair of normally ground engaging drive wheels on the supportframe rotatable about transverse axes, the drive wheels positioned belowthe seat portion and forward of the back portion; motor means forrotatably driving each of the drive wheels to provide powered movementof the wheelchair, the respective motor means supported on oppositesides of the frame; controller means for controlling the driving of thedrive wheels by their respective motors; at least one battery forsupplying power to the at least one motor, the battery supported on theframe; at least one normally ground-engaging castor wheel connected tothe frame behind the drive wheels and rearward of the back of the seat;a pair of anti-tip idler wheels, each said anti-tip wheel positionedforward of the drive wheels and the seat, on opposite sides of the frameand off the ground when the drive wheels and the castor wheel are intheir normal ground-engaging position on level ground, the anti-tipwheels comprising all of the wheels positioned forward of the drivewheels; a resilient mounting for each said anti-tip wheels, each saidmounting attached at a rear end to the frame and the anti-tip wheelrotatably supported at a front end, the anti-tip wheel being movableupwardly in a substantially arcuate motion relative to and forward ofthe attachment of the mounting to the frame, the upward movement beingresisted by the resilient mounting in the event of the wheel chairpitching forward and in the event of either of the anti-tip wheelscontacting an obstacle; and an adjustable foot rest for supporting thefeet of an occupant when positioned in the seat footrest supported onthe frame forward of the seat and between the anti-tip wheels.
 10. Thewheelchair according to claim 9, wherein said support arm is pivotallyattached to the support frame at one location and said resilientmounting includes a spring attached to the support frame at a secondlocation, the resilient mounting adapted to permit deflection of theanti-tip idler wheel about the pivotal attachment, the spring adapted toresiliently compress in the event of such deflection and to bias theanti-tip idler wheel back to its non-deflected position.